External Inosculation as a Feature of Revascularization Occurs After Free Transplantation of Murine Liver Grafts

The induction of angiogenesis is essential for successful engraftment of freely transplanted cells or cellular composites. How to augment angiogenesis to ensure an appropriate viability of the grafts is still under investigation. This study evaluated the proangiogenic capability of different syngeneic free liver transplants and elucidated the origin of the newly formed vascular network via use of an eGFP+/eGFP− (enhanced green fluorescent protein) cross‐over design. Using intravital fluorescence microscopy, we found that neonatal and resected murine liver transplants implanted into dorsal skinfold chambers display a significantly enhanced vascularization compared to regular adult transplants. Immunohistochemically, less tissue hypoxia, apoptosis and macrophage infiltration was observed in the neonatal and resected transplants, which is in line with improved vascularization of those grafts. Additionally, electron microscopy revealed morphological hallmarks of liver cells. eGFP+ liver transplants implanted on eGFP− recipients displayed vascular sprouting from the grafts themselves and connection to the recipients` microvasculature, which also undergoes transient proangiogenic response. This process is described as external inosculation, with microvessels exhibiting a chimeric nature of the endothelial lining. These data collectively show that proliferative stimulation is taking effect on angiogenic properties of free transplants and might provide a novel tool for modulating the revascularization of free grafts. Neonatal liver sections or sections retrieved from regenerating post‐resective murine livers display a better transplant outcome and enhanced revascularization compared to sections from regular adult liver grafts, suggesting a potential role of the transplant condition to positively stimulate induction of angiogenesis, an essential requirement for hepatic free tissue transplantation.